1. Field of the Invention
The present invention relates to a display device and, more particularly, to a display device including plural colors of sub-pixels.
2. Description of the Related Art
These years, development of organic electroluminescence (EL) display devices has been under way. For example, adoption of organic EL display devices for mobile phones has been under consideration. Methods of driving organic EL display devices include a passive matrix driving method and an active matrix driving method. In the case of the passive driving method, scan electrodes and data electrodes are used, and thus pixels are driven by time-division. In the case of the active matrix driving method, thin film transistors (TFTs) are arranged respectively in pixels, and thus light emission of respective pixels is held throughout one vertical scan time period.
In the case of organic EL display devices, it has been known that substantial luminous efficiencies of organic EL elements respectively for colors are different from one color to another, and that life spans of the organic EL elements are dependent on electric current densities applied respectively to the organic EL elements. With regard to organic EL display devices, a larger electric current has to be applied to sub-pixels with a poorer luminous efficiency than that applied to the other sub-pixels with a better luminous efficiency for the purpose of obtaining predetermined luminance in each of the sub-pixels with the poorer luminous efficiency, in a case where all of these sub-pixels have an equal area of light emission. That is because luminous efficiencies of organic EL elements respectively for colors are different from one color to another in the aforementioned manner. This brings about a problem that the elements of the sub-pixels to which the larger electric current is applied become shorter in life span, and another problem that a display device as a whole accordingly becomes shorter in life span.
For the purpose of solving such problems, the following method has been proposed. In the case of this method, areas of light emission in sub-pixels respectively for the colors are different from one color to another depending on luminous efficiencies thereof Thereby, sub-pixels for any one of the colors have a life span almost equal to those of sub-pixels for the other colors. (See Japanese Patent Laid-open Official Gazette No. 2001-290441, for instance)
In addition, a method of expressing videos by use of four colors of red, green, blue and white has been proposed as a method of reducing power consumption of organic EL display devices (see Japanese Patent Laid-open Official Gazette No. 2004-334204, for instance). Organic EL elements are self-luminous elements. For this reason, organic EL elements start to consume power when the organic EL elements start to emit light. In other words, organic EL display devices display videos by causing individual pixels to emit light. Thus, no sooner do the organic EL display devices start to display a video than the power consumption occurs. For this reason, in the case where videos are expressed by use of the three colors of red, green and blue, electric current is consumed most when white is displayed by emitting the three colors at a time. To put it another way, for the purpose of reducing power consumption, it suffices that sub-pixels for the three colors are designed not to emit light at a time. With this taken into consideration, videos are designed to be displayed by use of four colors which are obtained by adding white to the three colors. Luminous efficiency of a sub-pixel for white is at least more than twice as high as substantial luminous efficiency of each of sub-pixels respectively for red and blue. In addition, white is designed to be displayed by causing sub-pixels for white to emit light. Display of white consumes power most among displays respectively of the four colors. These designs make it possible to check sub-pixels for the three colors from emitting light, and to accordingly reduce power consumption. (see US Patent Published Application No. 20020186214, for instance)
With regard to conventional organic EL display devices each including sub-pixels for the three different colors, several types of circuit layouts have been known. A stripe layout has been known as the most generally-used circuit layout. In the case of the stripe layout, sub-pixels for the colors are arrayed in lines. Scan lines and data lines are provided. The data lines are arranged in a way that the data lines are orthogonal to the scan lines. For the purpose of driving active matrix organic EL display devices with the stripe layout, voltage is applied to the scan lines, and thus sub-pixels are selected. In addition, luminance levels respectively of the sub-pixels are controlled by use of voltage signals which are stored in the data lines.
A delta layout has been known as a second type of circuit layout. In the case of the delta layout, sub-pixels are laid out in triangular patterns instead of in lines. The delta layout makes sub-pixels respectively for the three colors closer to each other than the stripe layout does. For this reason, in many cases, the delta layout provides viewers with more desirable appearance than the stripe layout does. A method of applying such a circuit layout to organic EL display devices including sub-pixels for the four colors has been proposed (see Japanese Patent Laid-open Official Gazette No. 2004-334204, for example).
Particularly in a case where organic EL display devices are applied to mobile phones, it is strongly demanded that power consumption should be reduced. For this reason, the method of expressing videos by use of the four colors of red, green, blue and white, which has been disclosed in Japanese Paten Laid-open Official Gazette No. 2004-334204, is effective for the application of organic EL display devices to mobile phones. However, the present inventors have found that, in a case where a technique disclosed in Japanese Patent Laid-open Official Gazette No. 2001-290441 is applied to organic EL display devices which display videos by use of the four color of red, green, blue and white, the following problem is brought about. The problem is that life spans of sub-pixels are different from one color to another in a case where areas of light emission in sub-pixels respectively for the four colors are made different from one another depending on luminous efficiencies of the sub-pixels by applying the technique.
Furthermore, for the purpose of improving visibility of this type of organic EL display devices using the four colors, consideration needs to be paid lest sub-pixels should be arranged lopsidedly for each color.
With these conditions taken into consideration, the present invention has been made. An object of the present invention is to provide a technique which enables life span of organic EL display devices to be extended. Another object of the present invention is to provide a technique which realizes organic EL display devices with a better visibility.